The present invention relates to battery separators and electric batteries.
The term "battery" as pertaining to electric batteries is used herein to denote one or more electric cells connected together to convert chemical energy into electrical energy. Batteries are used to power a variety of devices including radios, toys, hearing aids and portable equipment. An "electric cell" is a device for converting chemical energy into electric energy. Dry cell batteries have an electrolyte made nonspillable by use of an absorbent material. Dry cell batteries are also known as "LeClanche" cells after George LeClanche who received a French patent in 1866 for an electric cell having a zinc electrode and a MnO.sup.2 coated carbon electrode in a nonspillable (hence the term "dry") electrolyte of ammoniurn chloride paste. By the 1960s other electrode systems including Ag/Zn, HgO/Zn and alkaline MnO.sub.2 /Zn cells were in use.
All batteries have at least one anode and one cathode separated by electrolyte and preferably a battery separator. "Battery separators" are physical barriers interposed between the anode and the cathode which prevent physical contact therebetween. Battery separators must be permeable to electrons and/or ions.
A variety of materials have been used as battery separators. Various dry cell and storage batteries have employed wheat flour and cornstarch paste, paper, wood veneer, hard rubber, porous rubber, celluloid, glass mats, regenerated cellulose and fiber-reinforced regenerated cellulose (sausage casings). A variety of materials have been explored for use as battery separators including polyvinyl alcohol, methyl cellulose, polypropylene, fiberglass, and crosslinked methacrylic acid grafted polyethylene. These separators are used to separate the positive and negative electrodes of a cell to prevent short circuits. Separators should distribute and retain electrolyte between the electrodes while preventing dendritic growths or soluble products from shorting the cell or migrating to an opposing electrode. Desirably, separators will: be stable in the cell environment resisting degradation by cell media; permit conduction across the separator of current transferring ions or charges; be capable of operation under conditions of use including desired operating temperatures, pressures, and forces; and be easily and economically fabricated into electric cells.
Battery separators have been used almost from the beginning of electric cell and battery development. Felted cloth, strips of rubber, thin wood, plastic, impregnated paper, microporous poly(vinylchloride), and woven fabrics of cotton or nylon have been used. Sealed cell batteries often use separators which would absorb all available electrolyte. Generally these absorbent separators are nonwoven. The earliest absorbent separators were cellulosic and later, resin bonded paper and polyamide based nonwovens were also used. Sterilizable nonwoven fabrics of polypropylene have also been used. Ag/Zn batteries have used cellulose fiber reinforced casing type separator since the 1960s.
Regenerated cellulose film (cellophane) has also been used as a battery separator, e.g. for Ag/Zn batteries. Disadvantageously, it suffers from a low electrolyte absorption rate. Noncellulosic nonwovens have also been laminated to cellulose films using adhesives to produce separators having high electrolyte absorbance and a fast absorption rate. Nonwoven polyamides, poly(vinylalcohol) (PVOH), acrylonitrile-vinyl chloride copolymer, polyesters, and polypropylenes have all been used as battery separators. Blends of PVOH with cellulose fibers have also been used as described in the article "Manufacture and Use of Nonwoven Separators", Batteries International, pp. 44, 45 and 48, October, 1995, which article is hereby incorporated by reference in its entirety. Disadvantageously, laminate adhesives may interfere with electrolyte permeability across the separator and transfer of electrons and/or ions may be hindered causing increased resistance and lower voltage. Also, laminates using adhesives including laminates held together with low amounts of adhesive or adhesives chosen to minimize resistance and transfer hindrance are subject to delamination which leads to shorting and early battery failure.